I just went through the same process with Goiot hatches. They did not quote availability, only price which was £214 per lens for their T44 hatch (424x424mm) which seemed a lot.
There are lots of online suppliers but one called rather appropriately, Cutmyplastic can supply 10mm tinted cut to the right size with radius corners and square holes for the handles for just under £60 each.
Only one of mine is in a poor state with crazing but as the others have a little too, and with a view to colour matching I think I’ll buy 3 of them at that price.
The key thing is getting an accurate drawing and of course this only works if the hatch is flat with no dome effect, but I suspect the Lewmar ones are flat too.
Getting the old lens out will be the tricky bit.